Method and device for producing radio frequency waves

ABSTRACT

The invention relates to a method for producing radio frequency waves, whereby a pulse laser for producing light pulses having a predetermined spectrum of frequency modes and a predetermined recurrence frequency is operated. The light pulses of the pulse laser are detected by means of a detector device, and corresponding electrical output signals forming the radio frequency signals are produced. Said pulse laser is actuated in a stabilized manner by means of an optical reference signal in such a way that the recurrence frequency of the light pulse is fixed. The invention also relates to a radio frequency generator for implementing said method.

DESCRIPTION

The invention concerns a method for the producing of radio frequencywaves, especially, a procedure for the generating of electrical signalsof said radio frequency, which can be made available as an output signalof a light detector, which receives laser pulses in the ultra shortrange. The invention also concerns a radio frequency generator, withwhich the said method can be realized and find utilization in theproduction of radio frequencies.

Radio frequency waves are electromagnetic waves of characteristic wavelengths, in the range of 0.01 to 100 cm and possess characteristicfrequencies (rf-frequencies) in the range of 1 MHz to 100 GHz. Radiofrequency waves of these properties are applied in signal transmissiontechnology, and in spectroscopic investigative methods such as solidbody spectroscopy or resonance spectroscopy. For the production of radiofrequency waves up to now, cavity resonators or quartz oscillators havebeen used, which could be tuned to desired output frequencies. Theconventional generators have the disadvantage of a limited frequencystability. Thus, for instance, quartz oscillators have a naturalstability limit due to growth faults of the piezo-electric quartz, whichis characterized by a relative frequency stability of 10⁻¹³ (in onesecond). This stability can be improved by applying special measures.For example, with helium cooled, sapphire-oscillators, a relativestability of 10⁻¹⁴ (in one second) can be achieved. This, however,requires a complex equipment, which, for most practical uses, isunacceptable.

Thus the purpose of the invention is to bring about an improved methodfor the generation of radio frequencies, in which the radio frequencywaves possess a greater stability and which can be achieved by arelatively simply constructed generator. The invented method shouldenable a frequency production with a relative stability of at least10⁻¹³. A second purpose of the invention is to make available animproved radio frequency generator, which can be applied for theexecution of the method.

The invented radio frequency generator should characterize itselfespecially by a high frequency stability and have a compactconstruction, which is robust and easily maintained.

These purposes are achieved by a method and a radio frequency generatorwith the respective features of claim 1 and claim 9. Advantageousembodiments and applications of the invention become evident in thesubordinate claims.

The fundamental concept of the invention, is to produce radio frequencywaves of at least one radio frequency, which corresponds to a separationfrom the frequency modes of the spectrum of short laser-light pulses.The light pulses are produced by a pulse-laser, the repetition rate ofwhich is stabilized, and directed to a detector apparatus. In theelectrical output signal of the detector apparatus, are to be foundfrequency components corresponding to the mode-separations in thefrequency spectrum. The repetition frequency stabilization of the pulselaser is effected by phase coupling of at least one frequency mode ofthe light pulse of the pulse laser with at least one highly stablereference frequency, or by the capture of an optical reference signal. Aparticularly high degree of stability of the radio frequency isachieved, when not only the repetition of the pulse laser, but also theso called offset frequency (see below) of the frequency components inthe mode-spectrum are stabilized.

In accord with preferred embodiments of the invention, a referencesignal is formed by a stabilized reference laser or, under certainconditions, by an incited atomic transition.

The object of the invention is also to create:

-   -   a radio frequency generator with a pulse laser for the        production of light pulses,    -   a detector apparatus for the generation of an electrical output        signal in radio frequencies and    -   an apparatus for the stabilization of the pulse laser,        with which, at least the repetition frequency, or optionally the        offset frequency, can determine the light pulse.

The invention possesses the following advantages. Since the radiofrequency is derived from at least one generator frequency, which isfixed by means of phase coupling with a stabilized optical referencefrequency or a control on the basis of an optical reference signal, thenthe radio frequency can be produced with relative stability, whichcorresponds to the relative stability of the reference signal. Opticalreference frequencies with relative stability can be produced withrelative stabilities better than 10⁻¹⁴. The construction of the inventedradio frequency generator is compact, and, especially can be implementedas a mobile, low-maintenance system by the use of a diode-pumped solidstate laser or a ring laser by employing a pulse laser for pulsegeneration. With the invention, simultaneously there is achieved both anincrease in the stability of the frequency as well as a diminishing ofthe cost of the apparatus in the production of radio frequencies.

Further advantages and details become evident from the description fromthe attached drawing.

FIG. 1 shows a schematic general presentation of an invented radiofrequency generator.

The Characteristics of an Ultra Short Light Pulse

The method of production of ultra short, laser-light pulses, whichmethod has been known for some 70 years, (ultra short referring to alight pulse with a characteristic duration of pulse in the ns to the fsrange) is based on the so-called mode-synchronization. In a lasermedium, with sufficient breadth of band of the laser transition in theresonator, many inherent vibrations can be incited at variousfrequencies. If, by means of an appropriate mechanism placed between theinherent vibrations, a firm phase relationship is (mode-synchronization)achieved, then an occurrence of the emission of short light pulses isbrought about with a time-duration of T, which is equal to the quotientsof double resonance length and average running speed of the pulse and aspectral combination corresponding to those optical frequencies whichare incited in the resonator and are contributory to the formation ofpulses.

Upon the construction of the curve of the intensity of the pulse formedlaser radiation in the frequency cavity, there is activated a modespectrum (or frequency comb) which, by means of δ-similar functions isformed by the optical frequencies contributing to each pulse and theincorporation of this within the band width of the laser transition lieswithin the laser medium. The breadth of that which is incorporated is,essentially, in inverse proportionality to the duration of the pulse.Each frequency contribution to a frequency comb of this kind will bedesignated here as “Frequency Mode M”.

The frequency separation of the elements of the frequency combcorrespond to the longitudinal, laser mode, integer multiple of thepulse-repetition frequency, namely f_(r)=τ⁻¹ (repetition rate). The combstructure of fs-pulses in the frequency cavity for instance, isdescribed in “Femtosecond Laser Pulses”, (Publisher C. Rulliere,Springer Verlag, Berlin 1998). The frequency components, which areseparated by integer multiples of the repetitive frequency f_(x) do notpermit themselves, in their absolute frequency state, to be presented asintegral multiples (n) of the repetition frequency, but by the sum(n·f_(r)+f_(o)) which is “n” times the repetitive frequency f_(r) plusthe offset—or phase-slip frequency f_(o)—which, for all frequencycomponents, is the same value, corresponding to the quotients from therespective phase differences from pulse to pulse by the rotation time(2π)τ.

The repetitive frequency of the light pulse, and therewith the frequencyseparation of the Modes, lies in the frequency range of radiofrequencies. If the light pulses are captured by a detector apparatus,which, in accord with the detected light intensity, itself generatesoutput signals, then the output signals contain frequency componentsagreeing with the individual frequency modes, that is, oscillatoryfrequencies between the frequency modes, which precisely represent thedesired radio frequencies. In accord with the invention, it is nowpossible to stabilize the pulse repetition frequency of the pulse laserwith reference to an optical frequency standard, as will be describedbelow.

Radio Frequency Generator

FIG. 1 shows, schematically, the construction of an invented radiofrequency generator 100 with:

-   -   a pulse laser 10,    -   a first detector apparatus 20,    -   and a circuit 30 for repetitive frequency stabilization.

There is also an optional apparatus 40 presented for offsetstabilization. The apparatus 40 offers no compelling feature tocharacterize the invention, but does present an additional improvementof the radio frequency stability. The pulse laser 10 can be made as anyconventional pulse laser with a typical pulse duration in thenanosecond, femtosecond range. The pulse laser 10, for example, can bedesigned as a Titanium Sapphire Laser (such as “Coherent Mira 900”,pulse length 73 femtoseconds, repetitive frequency f_(r)=75 MHz), or aring laser, or a diode pumped, solid state laser (such as aChromium-Lithium-SAF-Laser). The pulse laser 10, preferably, is acompact. portable, battery driven apparatus.

The pulse laser 10 is optionally connected with a device for thebroadening of the laser pulse by means of self-phase modulation (drawnin with dashed lines, reference number 11). The device 11, designed forpulse broadening, is, for example, an optical Single-Mode-Fiber. Theself phase modulation is described by K. Imai in “IEEE Journal ofQuantum Electronics”, Vol. 34, 1998, page 54ff. A particularly strongbroadening of the mode comb is achieved by the use of structured opticalfibers, which possess a fiber core and an axially running, concentric,thin air channel enclosing said fiber core. For description, see D.Mogilevetsev at al., in “Optics Letters” Vol. 23, 1998, page 1662ff,and/or T. A. Burks in “Optics Letters” Vol. 22, 1997, page 961ff, oragain T. A. Burks in “IEEE Photonics Letters”, Vol. 11, 1999, page647ff.

The detector 20 is a light sensitive element, such as, for example, aphoto-diode or a photo-multiplier. A portion of the output pulse of thepulse laser 10, (that is to say, the device 11) is deflected through thepartially transparent mirror 12 onto the first detector 20, the outputsignal (rf) whereof forms the desired radio frequency waves, or containsradio frequency. Further apparatuses 21 can connectedly follow thedetector 20 for the purpose of signal formation. Such signaling couldbe, for example, a filtering of the output signal and/or can entail anamplification thereof. For the amplification of an output signal, aphase coupling with a conventional microwave oscillator (not shown) canbe provided on the output signal of the detector 20 or a conventionalsemiconductor can be installed. The phase coupling is carried out in amanner analogous to the explained control circuits for the stabilizationof lasers.

The circuit 30 for frequency stabilization of the pulse laser 10includes a second detector 31, a reference frequency generator 32 forthe production of an optical reference signal and a control amplifier33.

The second detector 31 is likewise a light sensitive element (forexample, a photo detector or a photo multiplier).

The reference frequency generator 32 is generally designed to assure theavailability of a light signal with at least one frequency component,the frequency stability of which, is at least as high, as the stabilityof the radio frequencies to be generated should be. In accord with eachembodiment of the invention, the reference generator can be constructedwith a stabilized, continuous wave laser, a reference damping cell or anarrangement with a free atomic beam.

As an continuous wave laser with stabilized optical frequency thefollowing can be used:

-   -   a methane-stabilized helium-neon laser (output frequency 88 THz)        (if necessary with a non-linear frequency multiplier for        frequency matching at least at one mode of the light pulse of        the pulse laser 10,    -   or an iodine stabilized YAG Laser.

On the second detector 31 is carried out the simultaneous capture of amode of the laser pulse and the reference frequency. In the case of avery small deviation between the two frequencies, there arises anoscillating signal, which is emitted as an output signal of the seconddetector 31 by the control amplifier 33 to the pulse laser 10. The pulselaser 10 is equipped with a device for the control of the repetitivefrequency. This device is controlled in such a manner, that the saidoscillating signal of the second detector 31 vanishes, or represents apredetermined reference frequency. In this case the chosen frequencymode of the mode spectrum relative to the reference frequency of thereference frequency generator is set. In accord with this, the outputsignal of the first detector 20 is stabilized with the stability of thereference frequency generator 32.

Upon the construction of the reference frequency generator as areference damping cell, the pulse laser 10, is stabilized in referenceto the optical frequency of the atomic transition, which is excitedunder certain circumstances.

The reference damping cell is, for example, a cell which is temperaturecontrollable (for instance, iodine or rubidium cells). By means of theadjustment of the cell temperature, there is brought about in thereference cell a defined vapor pressure, which determines the locationand the breadth of the atomic transition.

What is provided, advantageously, is a Doppler-free optical excitationof the vapor corresponding to the 2-photon spectroscopy, to thesaturation spectroscopy or the polarisation spectroscopy.

The excitation is done, for example, with self reversed light pulses ofthe laser 10 running through the reference cell, as it has beendescribed by J. N. Eckstein, A. I. Ferguson and T. W. Hansch in“Physical Review Letters”, Vo. 40, 1978, p 847ff. The laser pulses arerun through the cell in two opposite directions, in such a manner, thatupon the collision of two light pulses, immediately a 2-photontransition can be incited, the total energy of which composes itself outof two partial amounts, which correspond exactly to two self-increasingfrequency components in the mode spectrum of the light pulse. Thisexcitation procedure enables a high resolution spectroscopy, since theDoppler extension of the observed transition is avoided. For theDoppler-free excitation, at a given distance from the reference cell, aplane mirror (not shown) is provided, with which the counter-runninglight pulses can be produced.

Alternative to the illustrated control on the basis of an oscillatorysignal, for instance a 2-photon-fluorescence from the reference cell canbe used directly as a adjusting signal for the control of the repetitionfrequency. The repetitive frequency is always exactly so set, that afluorescent signal will be captured by the detector 31.

The setting of the repetitive frequency in the pulse laser 10 is done ina known manner by means of the adjustment of the resonator length or ofthe capacity of the pump. A pump capacity control is preferred, sincethis is done electro-optically without any mechanical movement. Theenables quick changing of the repetitive frequency and thereby a greaterband width for the setting of the pulse laser 10.

For the optionally provided apparatus 40 for offset frequencystabilization, the laser pulse A which is emitted from the pulse laser10, and if necessary, subjected to self-phase modulation in theappropriate modulator 11, is subdivided, by beam splitting semi-mirrors42 to 45 into various spectral divisions B and C.

For this purpose, at least one of the mirrors 42, 43 or 45 is designedfor the spectral selective deflection of beam components of the lightpulse A. For instance, it is provided, that the portion B containscomponents of higher frequency of the mode spectrum and the portion Chas lower frequency components of the mode spectrum. For the makingavailable of a sufficiently strong oscillatory signal, at the thirddetector 41, the frequencies of the portions B and C are tuned to oneanother by a frequency multiplier or divider 46. In the case of the saidexample (C=lower frequency portion) the component 46 is a frequencymultiplier. The component 46 is a multiplier or a divider stage foroptical frequencies, as these are known from the state of thetechnology.

A particularly simple design becomes possible, in such a case in themodem spectrum the laser pulse bridges a complete frequency octave. Inthis case, the component 46 is an optical, non-linear crystal for thefrequency doubling (or halving). After the penetration through thefrequence multiplier (component 46), then there exists afrequency-shifted beam portion D. The beam portions B and D aresimultaneously directed to the detector 41. Upon the simultaneousincidence of the beam portions, then, at the detector 41, an electricaloutput signal is produced, in accord with the deviation of the frequencycomponent(s) of the beam parts B and D. This electrical output signalcarries a defined oscillatory frequency. The output signal is given tothe first control amplifier 47, which activates a known apparatus forthe setting of the offset frequency of the pulse laser. For instance,with the control amplifier 47, the introduction of a linear dispersionin the resonator of the pulse laser 10 is controlled, as it is describedin the not yet published PCT/EP00/02135 or as described in thepublication of T. Udem et al., in Physical Review Letters”, Vol. 82,1999, Page 3568ff. A pump capacity control for the setting of the offsetfrequency possessed indeed the advantage, that the control is carriedout electro-optically without mechanical movements. In this situation,the repetition frequency is regulated by means of the resonator.

In accord with a converse embodiment example, the portion B can containthe low frequency components and the portion C the higher frequencycomponents, whereby the component 46 can then be designed for frequencydivision. Alternatively, also the apparatus for the frequencymultiplication or division can be provided in any other branch of thebeam splitters 42 to 45.

In accord with yet another diversive change, the mirrors 42 to 45 can bereplaced by other, similarly active beam splitters, such as prisms.Additionally, in the presented course of the beams, the portions A, Band C can be provided with optical filters and, if required, be given atime-delay for the tuning of the incident penetration of the beamportions at the detector.

APPLICATIONS

The radio frequency generator 10, in accord with the invention, withadvantage, can be implemented into all applications, which are known inthe technologies of signaling, spectroscopy, and time measuring.Especial advantages arise in applications, in which a low phase noise isdesired, for instance, in radar technology, or as radio frequencystandards in time technology or spectroscopy.

The disclosed features in the foregoing description, the drawing and theclaims, can be meaningful just as well standing on their own as inoptional combinations for the realization of the invention in itsvarious embodiments.

1. A method for the production of radio frequency waves, comprising thesteps of: operating a pulse laser for the production of light pulseswith a predetermined spectrum of frequency modes and a predeterminedrepetitive frequency wherein the frequency modes of the spectrum havepredetermined mode distances; stabilizing the pulse laser with anoptical reference signal such that the repetitive frequency of the lightpulses is fixed; capturing light pulses of the pulse laser with a firstdetector; and generating corresponding electrical output signals, whichinclude radio frequency components corresponding to the mode distances,which radio frequency components form the radio frequency waves.
 2. Amethod in accord with claim 1, in which the frequency stabilization iscarried out by a control circuit, in which, the repetitive frequency ofthe pulse laser is adjusted, dependent upon an oscillatory signalcaptured by a second detector from at least one first frequency mode ofthe light pulse of the pulse laser, and from an optical referencefrequency, which forms the optical reference signal.
 3. A method inaccord with claim 2, wherein the optical reference frequency is producedby a stabilized reference laser.
 4. A method in accord with claim 2, inwhich the optical reference signal is produced with a reference dampingcell, in which an atomic transition is incited under predeterminedconditions.
 5. A method in accord with claim 1, wherein the frequencystabilization is executed by a control circuit in which the repetitivefrequency of the pulse laser is fixed, dependent upon a fluorescentsignal emanating from a reference damping cell which is captured by asecond detector, wherein an atomic transition is optically excited underpredetermined conditions.
 6. A method in accord with claim 1, wherein adiode-pumped, solid state laser, or a ring laser is employed as a pulselaser.
 7. A method in accord with claim 1, in which a broadening of thespectrum of the frequency modes of the light pulse is done by means ofself-phase modulation.
 8. A method in accord with claim 1, in which isprovided a stabilization of the offset-frequency of the frequencycomponents of the light pulse produced by the pulse laser.
 9. A radiofrequency generator, comprising: a pulse laser for the production oflight pulses with a predetermined spectrum of frequency modes and apredetermined repetitive frequency, wherein the frequency modes of thespectrum have predetermined mode distances; a first detector which isdesigned to produce from the light pulses of the pulse laser electricaloutput signals, which include radio frequency components correspondingto the mode distances, which radio frequency components form radiofrequency waves; and a circuit for frequency stabilization of the pulselaser relative to an optical reference signal with which the repetitivefrequency of the light pulses can be fixed.
 10. A radio frequencygenerator in accord with claim 9, in which the pulse laser has a devicefor the establishment of the repetitive frequency and the frequencystabilization circuit contains a second detector and a referencefrequency generator, wherein the second detector is designed for thepurpose of capturing simultaneously the light pulse of the pulse laserand the optical reference signal as a reference frequency, and toproduce an oscillatory signal, with which the equipment can be madecontrollable for the fixing of the repetitive frequency.
 11. A radiofrequency generator accord with claim 10, in which the referencefrequency generator is comprised of a stabilized continuous wave laser.12. A radio frequency generator in accord with claim 10, wherein thepulse laser possesses an element for the fixing of the repetitivefrequency and the frequency stabilization circuit has a second detectorand an optical reference cell for the production of the opticalreference signal, wherein the said second detector is designed for thepurpose of capturing the optical reference signal as a fluorescentsignal of the reference signal and to produce an output signal, withwhich the said element for the fixing of the repetitive frequency iscontrollable.
 13. A radio frequency generator in accord with claim 9, inwhich the pulse laser consists of a diode pumped solid state laser or aring laser.
 14. A radio frequency generator in accord with claim 9, inwhich an apparatus is provided for the broadening of the spectrum of thefrequency modes of the light pulse.
 15. A radio frequency generator inaccord with claim 14, in which the broadening apparatus is constructedof an optical fiber.
 16. A radio frequency generator in accord withclaim 9, in which is provided the circuit for the frequencystabilization of the pulse laser, whereby the frequency components ofthe light pulses can be fixed.